Property loss, personal injury and loss of life due to fire can often be minimized or avoided when smoke or heat detectors are employed to provide an alarm during the initial stages of a fire. Consequently, local law in many jurisdictions requires that smoke and heat detectors with alarms be provided in public and commercial buildings and private homes. This has led to the development of a wide variety of commercially available smoke and heat detectors which are battery operated or are wired into the electrical circuit for a building.
The least expensive gas, heat and smoke detectors are battery operated, and these units m ay be permanently installed upon walls or ceilings in an area to be protected. The batteries in these units must be periodically replaced, and generally such units provide an intermittent alarm signal when the battery charge drops below a predetermined level. U.S. Pat. No. 4,227,191 to Samuel Raber illustrates a battery powered smoke detector of this type. Unfortunately, homeowners are prone to remove a weak battery from a smoke detector to silence the low charge warning signal and then neglect to promptly replace the battery. Also, some local laws require that smoke and heat detectors be wired into the 110 V power supply for a building, and in such cases, battery powered units are unacceptable.
Smoke, gas, and heat detectors which are wired into a building power supply are normally permanent installations which must be installed by a qualified electrician, and these units operate continuously on the available house power supply. To preclude the likelihood that such permanently wired detectors will be rendered inoperative by a fire which rapidly disables the building power supply, it is conventional practice to provide these units with battery power from a battery which is recharged from the standard A.C. power supply. If this power supply is discontinued, the detector will continue to operate as long as battery power remains. Such a system is disclosed by U.S. Pat. No. 4,199,754 to R. W. Johnson and W. J. Raddi.
Fire detectors which are wired into a standard household A.C. power supply generally require a separate outlet box for each detector which is installed. In an attempt to eliminate this necessity, combination smoke detector and lamp structures have been provided which can be wired into a single outlet box such as the one installed for a conventional ceiling lamp. Such a structure is illustrated by U.S. Pat. No. 4,090,178 to E. G. Norris.
A common problem experienced with both battery powered and hard wired A.C. powered fire detectors is that they are, in all cases, a permanent installation. If such a detector is inadvertently placed too close to a stove or other source of normal and acceptable smoke or heat, the alarm will be triggered, and the detector must then be either moved to an acceptable location or deactivated and replaced by a new detector. Movement of a fire detector generally involves substantial inconvenience, for wall or ceiling surfaces which have been defaced by the detector mounting must be repaired and repainted, and often the services of an electrician are required to disconnect and reinstall the detector. To eliminate these problems, some attempts have been made to power detectors from existing light sockets, as disclosed by U.S. Pat. No. 4,365,237 to W. B. Knight. Although circuits of this type do operate effectively from a conventional bulb socket, they are unsuitable for use with ceiling sockets as they require the additional mounting surfaces which are provided with a table or floor lamp. This is due to the fact that the detector module constitutes a separately mounted module which is electrically powered from a lamp module but which is otherwise separate and distinct therefrom. In systems of this type, there is no electrical cooperation between the light and detector circuit which operate as independent elements.
The above problems which are inherent in conventional fire detectors, which are the most common type of detectors, are also prevalent in other types of electrically powered detectors.